There has been significant interest in increasing the share of renewable energy sources in the world energy landscape (Chu and Majumdar 2012). Associated technologies support the generation or capture of energy from carbon-neutral sources, storage so that the energy can be used when and where needed, and more efficient use. In discussions of alternatives available for power generation from renewable sources, solar energy conversion is prominent, given the vast amount of energy it can yield (peak irradiation of 7.5 kWh/m2/day, mean annual global irradiation of 8,372 terawatt hours [TWh]/yr), making it a potentially important candidate for a sizable portion of US energy.

This potential contrasts with its current relatively small portion of the global energy portfolio: 0.06 percent (IEA 2012; Zhang and Shen 2012). While economic factors account for a substantial part of the barrier to implementation, considerable technological challenges stem from the inherently intermittent nature of the solar generation process. Adoption of solar power generation will entail significant changes in operation of the power grid, as classical power generation plants will need to respond not only to changes in consumer demand but also to noncontrollable variations in energy generation.

BACKGROUND

One option to mitigate the intermittency of solar energy generation is the incorporation of energy storage capacity into the grid, so that fluctuations in energy generation are buffered and do not affect the operation of the electricity distribution channels. But large-scale implementation of energy storage faces both technological and economic hurdles requiring significant research and development.

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Artificial Solar Fuel Generators
Miguel A. Modestino
École Polytechnique Fédérale de Lausanne
Rachel A. Segalman
University of California, Berkeley
Lawrence Berkeley National Laboratories
There has been significant interest in increasing the share of renewable energy
sources in the world energy landscape (Chu and Majumdar 2012). Associated
technologies support the generation or capture of energy from carbon-neutral
sources, storage so that the energy can be used when and where needed, and more
efficient use. In discussions of alternatives available for power generation from
renewable sources, solar energy conversion is prominent, given the vast amount
of energy it can yield (peak irradiation of 7.5 kWh/m2/day, mean annual global
irradiation of 8,372 terawatt hours [TWh]/yr), making it a potentially important
candidate for a sizable portion of US energy.
This potential contrasts with its current relatively small portion of the global
energy portfolio: 0.06 percent (IEA 2012; Zhang and Shen 2012). While eco-
nomic factors account for a substantial part of the barrier to implementation,
considerable technological challenges stem from the inherently intermittent nature
of the solar generation process. Adoption of solar power generation will entail
significant changes in operation of the power grid, as classical power generation
plants will need to respond not only to changes in consumer demand but also to
non­ ontrollable variations in energy generation.
c
Background
One option to mitigate the intermittency of solar energy generation is the
incorporation of energy storage capacity into the grid, so that fluctuations in energy
generation are buffered and do not affect the operation of the electricity distribution
channels. But large-scale implementation of energy storage faces both technologi-
cal and economic hurdles requiring significant research and development.
97

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98 FRONTIERS OF ENGINEERING
Alternatively, one could take inspiration from nature, where energy is stored
in the form of chemical bonds. In the case of artificial photosynthesis, this means
the generation of fuels directly from solar energy.
Types of Solar Fuel Generators
Integrated energy capture and storage solutions such as solar fuel generators
have the potential to increase the fraction of renewables in the mix of energy
sources, and can apply to all sectors of energy consumption (industrial, com-
mercial, residential, and transportation) (Bard and Fox 1995; Chu and ­ ajumdar
M
2012; Concepcion et al. 2012; Faunce et al. 2013; Lewis and Nocera 2006; ­Nocera
2012). Integrated solar fuel generators are photoelectrochemical (PEC) cells
that can capture solar energy and catalytically convert low-energy reactants into
energy-dense fuels.
One category of solar fuel generators, water-splitting systems, take water
as a feed and produce hydrogen fuel and oxygen as byproduct. A general rep-
resentation of these systems is shown in Figure 1. Practical systems take water
and solar energy as inputs and produce output streams of hydrogen and oxygen
in a safe and scalable manner. In this way pure fuel streams can be collected and
used in electrochemical energy conversion devices (i.e., fuel cells) or in chemical
processes to synthesize or enhance the energy content of liquid fuels (e.g., the
Fischer-Tropsch process).
The concept of solar fuel generation can be extended to the electrochemical
reduction of CO2, which can yield carbon-containing fuels but represents a closed
cycle from the carbon perspective, making these new fuels truly carbon neutral.
Solar-driven CO2 reduction poses greater technical challenges because the number
of electron transfer steps in the reactions is higher, the concentration of CO2 in
electrolytes is generally low, and the diversity of products makes the necessary
separation more difficult.
Mechanics of Solar Fuel Generators
As shown in Figure 1, solar fuel generation begins with the absorption of light
to form charges that are used to drive oxidation and reduction reactions. These
three processes can be done in separate units—for example, using a photovoltaic
cell to generate electricity that powers an electrolyzer, which incorporates the
catalysts—or in a fully integrated device. Practical comparisons between these
two scenarios are largely dependent on possible gains in terms of economics and
flexibility of deployment.
A fully integrated water-splitting or solar hydrogen generator, as shown in
Figure 1, would consist of interconnected photovoltaic and catalytic units. Ideally,
the oxidation and reduction sites are physically separated so that the product (H 2
in the case of water splitting) is generated in a space different from the byproduct

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ARTIFICIAL SOLAR FUEL GENERATORS 99
FIGURE 1 (A) Solar fuel generators are composed of photoelectrochemical (PEC) devices
that can generate separate streams of fuels directly from sunlight (H2 and O2 in the case
of water splitting). (B) A PEC device contains photovoltaic units that absorb light and
move charges to catalytic centers, where electrochemical hydrogen and oxygen evolution
r
­eactions take place. In the diagram, the membrane component is used as the matrix for
PEC units and allows for both ion conduction (i.e., H+) and gas separation.

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100 FRONTIERS OF ENGINEERING
(O2 in this case). The size of the photovoltaic unit is generally set by the solar
absorption depth of the material and is on the order of microns to millimeters.
For a number of reasons it makes sense to have an array of photovoltaic units
held together by a mechanically robust membrane that separates the product gases
and shuttles ions from one catalyst site to the other. Under acidic conditions, water
will be dissociated into O2 and protons on the oxidation side of the membrane.
The protons will then be transported through the membrane to the reduction side,
where H2 will be evolved. In this way oxidation and reduction products will be
generated in separate regions of the membrane, preventing the need for further
separation.
In the case of operation under basic electrolytes, the processes are analogous
and the ionic current in the system is carried by OH− ions at steady state. The
incorporation of ion-conducting membranes is crucial for this type of operation,
as they provide transport pathways for charged intermediaries between the oxida-
tion and reduction sites and at the same time serve as a barrier for gas diffusion,
allowing the production of fuel in its pure form. Achieving this configuration can
be simple for macroscopic units, but for micrometer- to nanometer-scale (i.e.,
mesoscale) systems significant advances are required in terms of both membrane
and PEC unit self-assembly.
Progress on these technological options depends on research and develop-
ment currently under way in academic, government, and industrial laboratories.
In this article, we discuss aspects of an integrated system that are the focus of
current exploration and development. The sections below touch on some of the
advances and challenges in achieving practical solar fuel generators, implications
for mesoscale assemblies and for membranes used in these systems, and overall
system design considerations. Throughout, we describe current research as well
as specific areas that require further study to enable progress in this area.
Solar Fuel Generation Systems
Since the first demonstration of solar-driven water splitting by Fujishima
and Honda (1972), the prospect of using PEC cells for solar fuel generation has
motivated the quest for components and integrated systems that can continuously
and robustly produce hydrogen fuels directly from sunlight. Over the past 40 years
many studies have attempted to tackle parts of the problem, and fuel-generating
systems have reached solar hydrogen generation efficiencies of up to 18 percent
(Peharz et al. 2007).
But solar hydrogen generation units fall short in satisfying stability and
cost-effectiveness requirements. Some high-efficiency systems rely on III-V
multijunction photovoltaic components that have prohibitively high costs and
serious photocorrosion challenges at the interface between the semiconductor
and the electrolyte (Khaselev and Turner 1998; Khaselev et al. 2001; Peharz
et al. 2007).

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ARTIFICIAL SOLAR FUEL GENERATORS 101
Other systems, based on silicon light-absorbing components, including
earth-abundant catalysts, face significant stability problems when operated under
basic or acidic electrolytes. Recently, however, Nocera’s group at the Massachu-
setts Institute of Technology demonstrated integrated systems that incorporate
earth-abundant components that can stably operate under buffered electrolytes at
moderate pH (Reece et al. 2011). This promising demonstration can open avenues
for the implementation of cost-effective solar hydrogen generators, but important
challenges for the management of ion and mass transport remain, largely based
on the need to separate the gaseous products while providing pathways for steady-
state ion conduction (Haussener et al. 2012; Hernandez-Pagan et al. 2012).
When systems are operated at moderate pH regimes, the low concentra-
tion of proton or hydroxide conduction results in high solution resistance for
these ions, and most of the ionic current is carried by supporting ions pres-
ent in the solution (i.e., ions dissociated from buffer molecules). Under these
circumstances, as the conducting ions are not part of the electrode reactions,
concentration gradients will evolve and the overall system will not be able to
operate continuously.
Efficient solar hydrogen generation would represent a large step to increase
the share of renewable fuel sources but implementation would be challenging as
current infrastructure is based on liquid carbon–based fuels.
An alternative to solar water splitting lies in the direct reduction of CO2 for
the generation of liquid carbon–containing fuels (Gattrell et al. 2007; Kondratenko
et al. 2013; Lewis and Nocera 2006; Olah et al. 2008). Notwithstanding consid-
erable research in this field, challenges persist because requirements for catalyst
­selectivity, CO2 absorption, and product separation are quite stringent.
Last, the technoeconomic aspects of solar fuel generators are crucial for the
achievement of deployable systems. The US Department of Energy has set the price
of hydrogen produced at less than $4/kg, which imposes bounds on the material
systems and configurations that are implementable (Saur and Ainscough 2011).
Few reports have tackled these aspects or provided guidance to achieve this price
point (e.g., James et al. 2009; Pinaud et al. 2013).
As both the scientific and engineering aspects of artificial photosynthesis
devices mature, a better understanding of the challenges to fabricate cost-effective
solar fuel generators will be critical for their deployment.
Mesoscale Building Blocks for
Artificial Photosynthesis Systems
The examples cited above represent initial attempts at developing integrated
devices that can produce hydrogen fuels directly from the sun, and they all rely
on macroscopic PEC units arranged such that ion transport involves a liquid elec-
trolyte. Under concentrated electrolyte conditions (~1 M), ion transport does not
provide significant resistance if the ionic pathway is less than a few centimeters

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102 FRONTIERS OF ENGINEERING
(Haussener et al. 2012). Furthermore, if the ionic conductivity of electrolyte is
lowered, or for operation of systems under water vapor (Spurgeon and Lewis
2011), it is highly desirable to develop PEC units with dimensions in the micro- or
nanometer range so that ions have to migrate only small distances.
Several mesoscale building blocks for PEC units have been developed.
Complex nanocrystal structures (e.g., nanorods, nanowires) can be synthesized
in solution (Amirav and Alivisatos 2010; Dukovic et al. 2008; Sun et al. 2011,
2013) and have shown promising performance in terms of hydrogen evolution.
Methods for arranging these nanostructures into architectures that enable oxida-
tion and reduction reactions to occur at separate locations depend on the shape,
dimensions, and self-assembly characteristics of the particles.
For long semiconducting nanowire systems, large surface area mats permit
a percolated network of wires to act as a self-standing water-splitting membrane
(Sun et al. 2011). And for nanorod-based systems, self-assembly techniques are
required to achieve architectures resembling that shown in Figure 1 (Baker et al.
2010; Baranov et al. 2010; Gupta et al. 2006; Ryan et al. 2006).
Although these self-assembly techniques have demonstrated the fabrication
of large-scale vertically aligned nanorod arrays from solution, it is not clear how
to obtain preferential directionality of the ends of asymmetric water-splitting
nanorods (Amirav and Alivisatos 2010).
As an alternative to solution-based methods, photocatalytic units can be
directly grown via vapor-liquid-solid deposition methods so that the resulting
arrays have the desired directionality. The development of silicon-based microwire
arrays is an example of such a strategy and can lead to large-area coverage of the
photoactive components that can then be incorporated into ion-conducting mem-
branes (Boettcher et al. 2010; Maiolo et al. 2007; Plass et al. 2009; Spurgeon et
al. 2011). These systems have many advantages over planar PEC devices because
they can absorb nearly all the incident light with only a small fraction of areal
coverage (Kelzenberg et al. 2010) and each microwire in the arrays acts as an
independent unit, largely alleviating stability constraints.
The incorporation of mesoscale PEC units into fully functional solar hydro-
gen generators represents a promising alternative to overcome the technological
challenges that prevent deployment, and so a great deal of research is being
conducted in this area.
Membrane Materials for Artificial Photosynthesis
Membranes in solar hydrogen generators serve two basic functions: to pro-
vide pathways for ion conduction and to keep gaseous products separated (as
shown in Figure 2). Ion-conducting membranes, which have been investigated for
several decades, are important components not only in artificial photosynthesis
applications but also in a variety of energy conversion devices (Walter et al. 2010;
Zhang and Shen 2012). The fundamental similarities in membrane requirements

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ARTIFICIAL SOLAR FUEL GENERATORS 103
FIGURE 2 Diagram of membrane material used for solar water splitting. These materials
contain conductive domains capable of transporting ions across the membrane (positive
or negative), while preventing crossover of the gases produced.
between solar fuel devices, hydrogen fuel cells, and electrolyzers suggest an exist-
ing set of candidate materials.
In the case of artificial photosynthesis applications, the operating current
density is dictated by the solar absorption rate and is relatively low when com-
pared to the requirements for other similar devices, but very sensitive to crossover
due to the relatively small quantity of product. Moreover, the presence of a large
number of interfaces between the polymer and inorganic PEC components can
severely affect the structure and transport properties of common nanostructured
fuel cell membrane materials. Perfluorosulfonic acid (PFSA) ionomer membranes
(e.g., Nafion®) are the most prominent alternatives for proton conduction, given
their high ionic conductivity and remarkable chemical and structural stability.
With artificial photosynthesis membranes, high levels of conductivity are not
required and the emphasis should instead be on the balance between the ionic and
gas transport properties of materials. The development of ion-conducting block
copolymers (BCPs) represents a promising route to decouple these two properties,
as different blocks can be designed to provide complementary structural and gas
barrier properties as well as ionic conductivity.
Furthermore, properties of BCP systems can be easily tuned and optimized
by altering the molecular weight and volume fraction of each phase (­ eckham P
and Holdcroft 2010). BCP membranes based on blends with ionic liquids
(ILs) and polymerized ILs (PILs) are characterized by good ionic conductivity and
­tenability (Bara et al. 2008, 2009; Gu and Lodge 2011; Gwee et al. 2010; Hoarfrost
­
and Segalman 2011, 2012; Lu et al. 2009; Mecerreyes 2011; Simone and Lodge
2009). Recent work has demonstrated the potential of PIL BCP ­mate­rials for ­tuning

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104 FRONTIERS OF ENGINEERING
transport properties in membranes used for solar fuel applications (Schneider et
al. 2013; Sudre et al. 2013).
System Design Considerations
All the components of a solar fuel generator system need to operate stably
and perform efficiently under the same conditions (i.e., temperature, electrolyte
selection). Additionally, the photovoltage generated by the light-absorbing units
needs to be sufficient to support the water-splitting reaction (1.23 V), the catalyst
overpotential requirement, the ohmic drop associated with transporting both elec-
trons and ions across the device, and any additional overpotential that may arise
from chemical potential differences (i.e., concentration overpotential). Further­
more, all the transport processes in the system need to occur in parallel so that the
electronic current matches the ionic current across reaction sites.
Several electrochemical modeling studies provide some guidance on the
optimal arrangements and dimensions of each of the components in an integrated
solar hydrogen generator (Berger and Newman 2013; Haussener et al. 2012;
Surendranath et al. 2012; Winkler et al. 2013). The output from the photovoltaic
component must match the electrochemical load from the catalytic and ion trans-
port components of the device. By controlling the dimensions and component
architecture, it is possible to optimize the performance of the device so that it
operates at near maximum possible efficiency (Jacobsson et al. 2013; Peharz et
al. 2007; Winkler et al. 2013).
Looking Ahead
Optimizing the topology of the components in a device can help overcome
some stability limitations, achieve operations under a wide range of conditions,
and increase overall efficiency. As new components become available, significant
work in this design area is necessary to understand what shape and form will lead
to optimization of cost, efficiency, and stability.
Acknowledgments
This material is based on work performed at the Joint Center for Artificial
Photosynthesis, a DOE Energy Innovation Hub, through the US Department of
Energy Office of Science under Award No. DE-SC0004993.
References
Amirav L, Alivisatos AP. 2010. Photocatalytic hydrogen production with tunable nanorod hetero­
structures. Journal of Physical Chemistry Letters 1(7):1051–1054.